1. Field of the Invention
The present invention relates to a data outputting apparatus for a digital magnetic recording system, or more in particular, to a data outputting apparatus for extracting sync patterns from a serial playback signal containing a plurality of data blocks segmented by the sync patterns, outputting a sync signal in accordance with the sync patterns, and outputting data converted in parallel and in synchronism with the sync signal.
2. Description of the Prior Art
In a magnetic tape system of helical scan type for writing and reading a digital signal, the read signal reproduced from the magnetic tape is a serial signal having a plurality of discontinuous data segments each containing a plurality of successive pairs of sync patterns and data blocks.
A data outputting apparatus extracts sync patterns from such a serial signal, generates and outputs a sync signal including a sync pulse for each sync pattern, and outputs parallel data in synchronism with and in parallel to the particular sync signal. This data outputting apparatus, even in the case of failure to extract a sync pattern, has the function of generating a protected sync pulse on the basis of the predetermined period .tau. of a sync pattern and preventing the sync pulse from dropping off from the sync signal. When a protected sync pulse is generated, the data outputting apparatus converts a serial signal into a parallel signal as a data signal in response to the phase of the protected sync pulse. Then, once a sync pattern is rightly extracted, a serial signal is rightly converted into a parallel signal and a right data signal can be output even when extraction of the sync pattern ends in failure. When extraction of the leading sync pattern of a leading data segment fails, however, the serial signal is not rightly converted into the parallel signal, and therefore the data in the leading data block disappears.
In order to prevent this inconvenience, first, a sync pattern is extracted from a serial signal, and a sync pulse is generated for each extracted sync pattern to provide a first sync signal. Also, the serial signal is delayed by a factor of an integral multiple of the period of the sync pattern, the sync pattern is extracted from the delayed serial signal, and a sync pulse is generated for each extracted sync pattern to thereby provide a second sync signal. The first sync signal and the second sync signal are ORed, and the resulting logic sum is used as a new sync signal. Such a technique is proposed by JP-A-63-90070. This conventional method is described with reference to FIGS. 8A to 8E. The protected sync pulse before right extraction of a sync pattern is not necessary and ignored by way of explanation.
FIG. 8A shows a serial signal S.sub.IN. In this serial signal S.sub.IN, DM.sub.a designates a dummy data block, SG.sub.a a leading data segment, DB.sub.i (i=0 to n) a data block, C.sub.i a sync pattern, and D.sub.i a data section. FIG. 8B shows a delayed serial signal DS.sub.IN. The signal DS.sub.IN is a serial signal S.sub.IN delayed by one predetermined period of a sync pattern. FIG. 8C shows a first sync signal CK including a sync pulse K.sub.j generated from the sync pattern C.sub.j (j=1 to 3) extracted from the serial signal S.sub.IN. Due to the failure to extract the sync pattern C.sub.0 of the leading data block DB.sub.0, the sync pulse K.sub.0 corresponding to the sync pattern C.sub.0 is lacking. FIG. 8D shows a second sync signal DC.sub.K including a sync pulse K.sub.j generated from the sync pattern C.sub.j extracted from the signal DS.sub.IN. In this case, too, extraction of the sync pattern C.sub.0 of the leading data block DB.sub.0 has failed, and therefore the sync pulse K.sub.0 corresponding to the sync pattern C.sub.0 is lacking. FIG. 8E shows a new sync signal NC.sub.K that has been obtained by logic addition of the first sync signal CK and the second sync signal NC.sub.K. The data block DB.sub.i is extracted from the signal DS.sub.IN on the basis of the sync signal K.sub.i of the new sync signal NC.sub.K, and the data contained in the data section D.sub.i is synchronized with the sync pulse of the new sync signal NC.sub.K and is output in parallel to the new sync signal NC.sub.K. The drop-off of the leading sync pulse K.sub.0 is thus prevented. As a result, the data D.sub.0 of the leading data block DB.sub.0 is also prevented from dropping off.
In the conventional apparatus, a serial signal is delayed by a factor of N times (N is a positive integer) of the period of a sync pattern, but a failure to extract a sync pattern for N' times (N' is an integer greater than or equal to N) of leading sync patterns makes it impossible to prevent the leading sync pulse from dropping off, leading to the problem of disappearance of the leading data. In the case where an attempt to extract the sync patterns C.sub.0, C.sub.1 in FIG. 8A has failed, for example, the sync pulses K.sub.0, K.sub.1 drop off from the first sync signal CK, and so do the sync pulses K.sub.0, K.sub.1 from the second sync signal NC.sub.K. Thus the sync pulse K.sub.0 also drops off from the new sync signal NC.sub.K which provides a logic sum, with the result that the data D.sub.0 disappears from the leading data block DB.sub.0.